A conventional hydraulic drive system for construction machines comprises, as disclosed in U.S. Pat. No. 4,967,557 by way of example, a hydraulic pump of variable displacement type, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from the hydraulic pump, a plurality of flow control valves for controlling respective flows of the hydraulic fluid supplied to the hydraulic actuators, a plurality of pressure compensating valves for controlling respective differential pressures across the flow control valves, and a pump controller for controlling a delivery rate of the hydraulic pump. The pump controller has a function of input torque limiting control with which the delivery rate of the hydraulic pump is controlled to reduce as a pump delivery pressure is increased and to increased as it is reduced. In the case that the construction machine is a hydraulic excavator, the plurality of actuators include ones such as a boom cylinder, arm cylinder, bucket cylinder and swing motor, which actuators drive working members such as a boom, arm, bucket and swing, respectively.
When one of the flow control valves is shifted by a control lever, the hydraulic fluid is supplied at the flow rate controlled by that flow control valve to the corresponding actuators such as the boom cylinder, arm cylinder or bucket cylinder, thereby carrying out work such as digging of sand and earth. During this work, when the load exerted on the actuator is large and the pump delivery pressure exceeds a predetermined value, the input torque limiting control of the pump controller takes places so that the pump delivery rate is reduced to prevent an engine from stalling.
In an attempt of carrying out a digging work by the hydraulic excavator thus arranged, when the digging work is performed on rocks, it may often occur that a finger tip of the bucket is caught by the rock and then slips off therefrom to bring about a sudden change from a condition under high load pressure due to the increased digging resistance into a condition of non-load. In the event of such a sudden change into the non-load condition, because the pump controller continues the input torque limiting control as mentioned above, the flow rate of the hydraulic fluid supplied to the actuator such as the bucket cylinder or the arm cylinder is abruptly changed. This tends to invite an inconvenience that the actuator speed is raised up beyond a required level and the bucket is excessively accelerated to strike against the rock intended by an operator to be next dug. The occurrence of such a collision imposes a heavy impact load on the hydraulic excavator body, hydraulic system and so forth, and thus remarkably cuts down on the service life of the hydraulic excavator as a structure. Further, the above collision transmits a heavy impact to an operating cab installed on the machine body as well, making the operator in the cab more fatigued.
The present invention has been made in view of the above-described state in the prior art, and has for its object to provide a hydraulic drive system for construction machines which can prevent an abrupt increase in the speed of a hydraulic actuator incidental to such an abrupt drop of the load as not caused during an ordinary work.